1. Field of the Invention
This invention relates generally to satellite receivers, and more particularly, to a method and apparatus for prefiltering a global positioning system receiver.
The United States government has placed into orbit a number of satellites as part of a global positioning system (GPS). A GPS receiver receives signals from several such satellites and can determine very accurate parameters, such as position, velocity, and time. There are both military and commercial uses. A primary military use is for a receiver in an aircraft or ship to constantly determine the position and velocity of the plane or ship. An example of a commercial use includes surveying and the accurate determination of the location of a fixed point or a distance between two fixed points, with a high degree of accuracy. Another example is the generation of a high accuracy timing reference.
In order to accomplish this, each satellite continually transmits two L-band signals. A receiver simultaneously detects the signals from several satellites and processes them to extract information from the signals in order to calculate the desired parameters, such as position, velocity or time. The United States government has adopted standards for these satellite transmissions so that others may utilize the satellite signals by building receivers for specific purposes. The satellite transmission standards are discussed in many technical articles and are set forth in detail by an "Interface Control Document" of Rockwell International Corporation, entitled "Navstar GPS Space Segment/Navigation User Interfaces", dated Sep. 26, 1984, as revised Dec. 19, 1986.
Briefly, each satellite transmits an L1 signal on a 1575.42 MHz carrier, usually expressed as 1540f.sub.0, where f.sub.0 =1.023 MHz. A second L2 signal transmitted by each satellite has a carrier frequency of 1227.6 MHz, or 1200f.sub.0.
Each of these signals is modulated in the satellite by at least one pseudo-random signal function that is unique to that satellite. This results in developing a spread spectrum signal that resists the effects of radio frequency noise or intentional jamming. It also allows the L-band signals from a number of satellites to be individually identified and separated in a receiver. One such pseudo-random function is a precision code ("P-code") that modulates both of the L1 and L2 carriers in the satellite. The P-code has a 10.23 MHz clock rate and thus causes the L1 and L2 signals to have a 20.46 MHz bandwidth. The P-code is seven days in length. In addition, the L1 signal of each satellite is modulated by a second pseudo-random function, or a unique clear acquisition code ("C/A-code"), having a 1.023 MHz clock rate and repeating its pattern every one millisecond, thus containing 1023 bits. Further, the L1 carrier is also modulated by a 50 bit-per-second navigational data stream that provides certain information of satellite identification, status and the like.
In a receiver, signals corresponding to the known pseudo-random functions are generated and aligned in phase with those modulated onto the satellite signals in the process of demodulating those signals. The phase of the carriers from each satellite being tracked is measured from the results of correlating each satellite signal with a locally generated pseudo-random function. The relative phase of carrier signals from a number of satellites is a measurement that is used by a receiver to calculate the desired end quantities of distance, velocity, time, etc. Since the P-code functions are to be classified by the United States government so that they can be used for military purposes only, commercial users of the GPS must work only with the C/A code pseudo-random function.
The GPS receiver industry has been plagued with many problems, including for example, (1) in unintentional jamming from other users of the RF spectrum; (2) in acquisition of data from low elevation satellites; and (3) in dual land GPS receivers, splitting the L1 and L2 signals without increasing the chances of being adversely impacted by (1) and (1) above, to name a few.
The accuracy with which positions are determined using GPS depends on two factors; the satellite configuration geometry and the measurement accuracy. The usual term for GPS measurement accuracy is the user equivalent range error (UERE), which represents the combined effect of ephemeris uncertainties, propagation errors, clock and timing errors, and receiver noise.
The effect of satellite configuration geometry is expressed by the dilution of precision (DOP) factor, which is the ratio of the positioning accuracy to the measurement accuracy, or EQU .sigma.=DOP..sigma..sub.0,
where .sigma..sub.0 is the measurement accuracy (standard deviation), and .sigma. is the positioning accuracy (e.g., standard deviation in one coordinate). DOP is a scalar representing the contribution of the configuration geometry to the positioning accuracy. There are many varieties of DOP, depending on what particular coordinate, or combinations of coordinates, one is considering the accuracies of. The more common DOPs are:
VDOP..sigma..sub.0 is the standard deviation in height (Vertical) PA1 HDOP..sigma..sub.0 is the accuracy in 2D Horizontal position PA1 PDOP..sigma..sub.0 is the accuracy in 3D Position PA1 TDOP..sigma..sub.0 is the standard deviation in Time PA1 HTDOP..sigma..sub.0 is the accuracy in Horizontal position and Time PA1 GDOP..sigma..sub.0 is the accuracy in 3D position and time (Geometrical).
The term DOP comes from the confidence ellipsoid, a way to quantify the accuracy of position. The standard deviation in one coordinate (i.e. height) is represented by the distance from the center to the surface of the ellipsoid along the local vertical direction (the height axis). According to the above equation, it is also equal to VDOP..sigma..sub.0. For horizontal positioning, in confidence ellipsoid terms, one generally expresses the "size" of the horizontal ellipse: one reasonable measure is square root of the sum of squares of the two axes of the horizontal ellipse. This is HDOP..sigma..sub.0. In general, any DOP is equivalent to the square root of the sum of the squares of the confidence region axes corresponding to the parameters being assessed.
The position accuracy is maximized using GPS, when one satellite is at the user's zenith (overhead satellite) and three others are separated by 120.degree. and are as low on the horizon (low elevation satellite) as permitted by the user's antenna elevation angle (maximize the horizontal cross-sectional area). Conversely, less accuracy in position is obtained when satellites are bunched together.
However, since the signals from low elevation satellites tend to be weaker and noisier than an overhead satellite, the performance or accuracy of a GPS receiver is hindered and the acquisition of reliable data from such satellites is more difficult. Accordingly, it is desirable to maximize the signal from low elevation satellites, while minimizing the noise in a GPS receiver.
It is generally held that the more loss in the front end of a multi-channel GPS receiver, i.e. in the antenna and splitter, the lower the received signal to noise ratio and the lower the position accuracy of such receiver will be. It is therefore desirable to minimize noise and signal loss in the front end of a GPS receiver for improved performance and position accuracy.
Applicant is not aware of any multi-channel GPS patents disclosing prefilters and splitters to reduce the noise figure in the front end of a GPS receiver before entering the radio frequency receiving section, while simultaneously splitting and delivering the signals to each channel with little or no signal loss or reflections. It is also extremely important to minimize the noise figure and signal loss in the radio frequency section of a GPS receiver for accurate position data.
In a multi-channel GPS receiver utilizing a single antenna, the signal from the antenna must be split to at least two channels. Applicant is not aware of any patents relating to two channel GPS receivers that prefilter the signal in the front end before entering the RF receiving section, with minimal unwanted RF reflections and loss therein, before entering such RF receiving section.
It is desirable to filter GPS receivers to address these problems.
2. Description of the Related Art Including Information Disclosed under 37 USC .sctn.1.97-99
Many methods and apparatus relating to global positioning system receivers have been disclosed. Typifying these are those listed below.
U.S. Pat. No. 4,928,106 discloses an improved GPS receiver. The receiver is formed in two major sections. The first is a radio frequency section that simultaneously receives the L-band signals from a plurality of satellites and develops low intermediate frequency signals within the capability of readily available digital circuits. The second is a digital processing section which receives the intermediate frequency signals, correlates them with the C/A code of each satellite whose signals are being processed, and provides measurements of the relative phase of each signal. Correlation with (demodulation by) the C/A-code pseudo random function is accomplished in the digital section, not in the radio frequency section of the receiver. The relative phase and other measurements are then used by a processor to calculate the desired end quantities, such as position, distance, velocity, time and the like. All clocks and timing signals used by both the radio frequency and digital processing sections of the receiver are mutually coherent, being derived from a common oscillator.
U.S. Pat. No. 4,445,118 discloses a navigation system, such as the GPS system, wherein the position coordinates of user terminals 14 are obtained by processing multiple signals transmitted by a constellation orbiting signals 16, an acquisition aiding signal generated by an earth-based control station 12 is relayed to user terminals via a geostationary satellite 10 to simplify user equipment. The aiding signal is FSK modulated on a reference channel slightly offset from a standard GPS channel. The aiding signal identifies satellites in view having best geometry and includes Doppler prediction data as well as GPS satellite coordinates and identification data associated with user terminals within an area being served by the control station 12 and relay satellite 10. The aiding signal is supposed to reduce user equipment by simplifying spread spectrum signal demodulation and reducing data processing functions previously carried out at the user terminals 14.
U.S. Pat. No. 4,468,793 discloses a global positioning system comprising an RF receiver for receiving L1, L2, P-code or C/A-code modulated frequency outputs from one or more space vehicles, a multiplexer connected to the receiver multiplexes the L1 and L2 signals to the receiver, and code and carrier tracking loops are connected to the receiver, each loop includes a plurality of filters, one for tracking lanosite dynamics and another for determining ionosphere effects on the L1 and L2 signals. Referring to FIG. 1b, a flip-flop 24 controls the switch 22 to admit alternately the L1 and L2 coded frequency signals to a first stage mixer 30 of the first stage of the two stage down conversion RF module 17.
U.S. Pat. No. 4,622,557 discloses a transdigitizer for relaying signals from global positioning system satellites. First, an RF stage comprising an antenna, filter and preamplifier receiver, filters and amplifies the 1575 MHz signals. Following the RF stage, a converter stage consisting of a bandpass filter, converts the GPS signal to a lower frequency. Then, an intermediate frequency comprising an IF amplifier, multiplier, bandpass link filter and limiter further amplifies and filters the signals to remove the effects of the signals. A final down converter converts the signal to a base band frequency and in a zero crossing detector, the signal is amplified and one bit quantitized. Finally, a local oscillator controls a frequency synthesizer to latch the signal from the zero crossing detector in a flip-flop, which in turn is used to control a quadraphase monitor, whose signals are amplified and transmitted out the transmit antenna.
U.S. Pat. No. 4,457,006 discloses a global positioning system receiver, having a biphase modulated radio frequency input signal applied to the front end of a double heterodyne receiver having a second intermediate frequency stage which operates in the audio frequency range. The audio output signal is phase locked to a one KHz reference signal and is applied to a microprocessor for processing via an interface circuit which includes an amplitude detector and a biphase detector. The microprocessor also controls the phase shifting of a pseudorandom noise code generator whose output is modulated with the output of a first intermediate frequency stage of the receiver.
U.S. Pat. No. 4,426,712 discloses a correlation system for a global positioning receiver, for receiving and interpreting data in the GPS including faster-than-real-time correlators for correlating the code portions of individual signals with matching codes stored in memory, thus creating a plurality of virtual channels for acquiring and tracking each visible satellite.
U.S. Pat. No. 4,359,733 discloses a satellite-base vehicle position determining system for determining the positions of a plurality of vehicles traveling on or above a defined sector of the earth's surface, which includes a transponder carried by each vehicle for transmitting a uniquely coded beacon signal in response to a general interrogation signal, at least three repeater-carrying satellites at spaced orbital locations above the earth for receiving and retransmitting the beacon signals produced by the vehicles, and a ground station for periodically transmitting the general interrogation signal and for receiving and processing the beacon signals retransmitted by the three satellites in order to determine vehicle position. In order to avoid signal overlap and equipment saturation at the ground station, each vehicle transponder includes means responsive to the general interrogation signal for inhibiting the transmission of further beacon signals by the transponder for a predetermined time interval following the response of the transponder to the general interrogation signal.
The following patents are directed to non-analogous radio frequency splitting methods or devices.
U.S. Pat. No. 4,902,991 discloses a radio frequency signal combining/sorting device which includes a plurality of filters, and a diplexer device connected to one side of input/output sides of each of the filters for combining/sorting signals, and includes a coupling device formed at the input/output ends of the filter connected side of the diplexer device, with the filters being formed at their side connected to the diplexer device, with openings for receiving the coupling device. In column 3, lines 47 et. seq. the electrical link of the individual transmission lines 22 from the junction point 24 including the coupling loops 25 is set at 1/4 wavelength, for example at an electrical angle .phi. shown in FIG. 2 of 90.degree., on the assumption that the channel filters 10 and 11 are not coupled with the duplexer means 12 at the required center frequency band of the transmitter multiplexer.
U.S. Pat. No. 5,068,629 discloses a nonreciprocal circuit element having a ferrite assembly which has a pair of ferrite members and a plurality of central conductors interposed between the ferrite members, and a dielectric substrate which has a earthing electrode formed on one of its faces and a plurality of impedance matching electrodes formed on the other face, and wherein a direct current magnetic field is applied to the ferrite members. The ferrite assembly and the dielectric substrate are stacked such that lead-out portions of the central conductors are, respectively, connected to the impedance matching electrodes, while earthing portions of the central conductors and the earthing electrode are grounded. In a preferred embodiment, the element is provided between a transmitter and a duplexer of a mobile telephone system.
U.S. Pat. No. 4,916,582 discloses an electronic component such as an inductor, a bandpass filter or a duplexer wherein an electronic component core and shielding electrode layers interpose intermediate layers made of a nonmetal material. The component is made by laminating intermediate layers, shielding electrode layers and protective layers in this order on both main surfaces of a electronic component core in order to form a laminated body, and a step of baking the laminated body.
U.S. Pat. No. 4,546,334 is directed to an electrical filter device in which cut-off spaces required in a casing of the device are reduced more so than in conventional arrangements, and which are free from unnecessary coupling and are constructed entirely by capacitor coupling for compact size.
None of the cited references disclose or suggest a method and apparatus for prefiltering a global positioning system receiver of this invention.
It is a desirable feature of the present invention to provide a global positioning receiver that is reliable, can be made for a low cost, is a low power consumer, and includes a simple receiver structure.
It is another desirable feature of the present invention to provide a receiver system that improves the accuracy of the ultimate quantities desired, such as position, velocity, and time.